Steady-state methods for measuring in-plane thermal conductivity of thin films for heat spreading applications.

The development of high thermal conductivity thin film materials for the thermal management of electronics requires accurate and precise methods for characterizing heat spreading capability, namely, in-plane thermal conductivity. However, due to the complex nature of thin film thermal property measurements, resolving the in-plane thermal conductivity of high thermal conductivity anisotropic thin films with high accuracy is particularly challenging. Capable transient techniques exist; however, they usually measure thermal diffusivity and require heat capacity and density to deduce thermal conductivity. Here, we present an explicit uncertainty analysis framework for accurately resolving in-plane thermal conductivity via two independent steady-state thermometry techniques: particle-assisted Raman thermometry and electrical resistance thermometry. Additionally, we establish error-based criteria to determine the limiting experimental conditions that permit the simplifying assumption of one-dimensional thermal conduction to further reduce thermal analysis. We demonstrate the accuracy and precision (<5% uncertainty) of both steady-state techniques through in-plane thermal conductivity measurements of anisotropic nanocrystalline diamond thin films.

Thermal Conductance of Poly(3-methylthiophene) Brushes.

A wide variety of recent work has demonstrated that the thermal conductivity of polymers can be improved dramatically through the alignment of polymer chains in the direction of heat transfer. Most of the polymeric samples exhibit high conductivity in either the axial direction of a fiber or in the in-plane direction of a thin film, while the most useful direction for thermal management is often the cross-plane direction of a film. Here we show poly(3-methylthiophene) brushes grafted from phosphonic acid monolayers using surface initiated polymerization can exhibit through-plane thermal conductivity greater than 2 W/(m K), a 6-fold increase compared to spin-coated poly(3-hexylthiophene) samples. The thickness of these films (10-40 nm) is somewhat less than that required in most applications, but the method demonstrates a route toward higher thermal conductivity in covalently grafted, aligned polymer films.

A carbon nanotube optical rectenna.

An optical rectenna--a device that directly converts free-propagating electromagnetic waves at optical frequencies to direct current--was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 PHz (switching speed on the order of 1 fs). Diodes operating at these frequencies are feasible if their capacitance is on the order of a few attofarads, but they remain extremely difficult to fabricate and to reliably couple to a nanoscale antenna. Here we demonstrate an optical rectenna by engineering metal-insulator-metal tunnel diodes, with a junction capacitance of ∼2 aF, at the tip of vertically aligned multiwalled carbon nanotubes (∼10 nm in diameter), which act as the antenna. Upon irradiation with visible and infrared light, we measure a d.c. open-circuit voltage and a short-circuit current that appear to be due to a rectification process (we account for a very small but quantifiable contribution from thermal effects). In contrast to recent reports of photodetection based on hot electron decay in a plasmonic nanoscale antenna, a coherent optical antenna field appears to be rectified directly in our devices, consistent with rectenna theory. Finally, power rectification is observed under simulated solar illumination, and there is no detectable change in diode performance after numerous current-voltage scans between 5 and 77 °C, indicating a potential for robust operation.

High thermal conductivity of chain-oriented amorphous polythiophene.

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

Carbon nanotube thermal interfaces enhanced with sprayed on nanoscale polymer coatings.

Vertical carbon nanotube (CNT) forests bonded at room temperature with sprayed on nanoscale polymer coatings are found by measurement to produce thermal resistances that are on a par with those of conventional metallic solders. These results are achieved by reducing the high contact resistance at CNT tips, which has hindered the development of high performance thermal interface materials based on CNTs. A spray coating process is developed for depositing nanoscale coatings of polystyrene and poly-3-hexylthiophene onto CNT forests, as a bonding agent that mitigates thermal resistance by enhancing the area available for heat transfer at CNT contacts. Resistances as low as 4.9 ± 0.3 mm(2) K W(-1) are achieved for the entire polymer coated CNT interface structure. The suitability of the spray coating process for large-scale implementation and the role of polymer and CNT forest thickness in determining the thermal resistance are also examined.

Regulated and unregulated emissions from highway heavy-duty diesel engines complying with U.S. Environmental Protection Agency 2007 emissions standards.

As part of the Advanced Collaborative Emissions Study (ACES), regulated and unregulated exhaust emissions from four different 2007 model year U.S. Environmental Protection Agency (EPA)-compliant heavy-duty highway diesel engines were measured on an engine dynamometer. The engines were equipped with exhaust high-efficiency catalyzed diesel particle filters (C-DPFs) that are actively regenerated or cleaned using the engine control module. Regulated emissions of carbon monoxide, nonmethane hydrocarbons, and particulate matter (PM) were on average 97, 89, and 86% lower than the 2007 EPA standard, respectively, and oxides of nitrogen (NOx) were on average 9% lower. Unregulated exhaust emissions of nitrogen dioxide (NO2) emissions were on, average 1.3 and 2.8 times higher than the NO, emissions reported in previous work using 1998- and 2004-technology engines, respectively. However, compared with other work performed on 1994- to 2004-technology engines, average emission reductions in the range of 71-99% were observed for a very comprehensive list of unregulated engine exhaust pollutants and air toxic contaminants that included metals and other elements, elemental carbon (EC), inorganic ions, and gas- and particle-phase volatile and semi-volatile organic carbon (OC) compounds. The low PM mass emitted from the 2007 technology ACES engines was composed mainly of sulfate (53%) and OC (30%), with a small fraction of EC (13%) and metals and other elements (4%). The fraction of EC is expected to remain small, regardless of engine operation, because of the presence of the high-efficiency C-DPF in the exhaust. This is different from typical PM composition of pre-2007 engines with EC in the range of 10-90%, depending on engine operation. Most of the particles emitted from the 2007 engines were mainly volatile nuclei mode in the sub-30-nm size range. An increase in volatile nanoparticles was observed during C-DPF active regeneration, during which the observed particle number was similar to that observed in emissions of pre-2007 engines. However, on average, when combining engine operation with and without active regeneration events, particle number emissions with the 2007 engines were 90% lower than the particle number emitted from a 2004-technology engine tested in an earlier program.